The present invention relates to the determination of heart function utilizing an impedance cardiograph, and particularly to such apparatus which determines the cardiac output by ensemble averaging thoracic impedance data measured during a number of cardiac cycles.
Impedance cardiography is a non-invasive technique for determining cardiac parameters in humans. Traditionally it has consisted of placing of two electrode bands circumferentially around the neck and two additional electrode bands around the thorax. A high frequency, low magnitude electric current was applied to the two outer electrode bands. The voltage changes between the inner two electrode bands were measured and correlated to the impedance of the patient. Changes in the baseline thoracic impedance (Z.sub.0) have been related to the changes of intravascular and extravascular fluids within the thorax. Thoracic impedance changes correlate closely with alterations of central blood volume. From the change of the value of a first derivative of the impedance, information concerning cardiac activity, such as cardiac output, can be obtained.
To obtain the cardiac output, the stroke volume representing the amount of blood being ejected during each cardiac cycle first must be computed. Stroke volume can be determined from the following equation disclosed by William G.
Kubicek et al. in U.S. Pat. No. 3,340,867: ##EQU1## where SV is the ventricular stroke volume, .rho. is the resistivity of blood at the excitation frequency, L is the shortest distance between the inner electrodes, Z.sub.0 is the average baseline impedance between the inner electrodes, T is the ventricular ejection time, and dZ/dt.sub.max is the magnitude of the first derivative of the thoracic impedance. The cardiac output then is computed by multiplying the stroke volume by the heart rate.
One of the problems encountered in using thoracic impedance to derive the stroke volume is that the thoracic impedance is also influenced by the effects of respiration. Similarly, if the patient is moving, as occurs during a stress test, the movement also interferes with the thoracic impedance, and the subsequent calculation of stroke volume and cardiac output. Previously thoracic impedance cardiography could only be performed on patients who were relatively motionless and apneic to avoid distorting the impedance data.
In addition, when this technique is applied to patients with severe cardiac dysfunction, the electrocardiogram signal and the thoracic impedance waveform may vary markedly from one cycle to another. A technique is required for determining whether a given set of data taken during the cardiac cycle is qualitatively sufficient from which to derive valid stroke volume and cardiac output results. Further, when such varying signals are to be averaged, a mechanism must be provided to normalize the signals so that deviations due to cardiac arrhythmia will not adversely affect the cardiac output computation.